Ulcerative colitis and Crohn's' disease, collectively referred to as Inflammatory Bowel disease (IBD), are chronic spontaneously relapsing disorders of the gastrointestinal tract. IBD is mediated at least in part by autoimmune mechanisms that are not completely understood. Studies of well-defined animal models of IBD have implicated several types of regulatory T cells, including CD4+CD25+ T cells, Tr1 and Th3 cells, in regulation of mucosal tolerance. However, none of these regulatory cells is known to naturally reside in the gastrointestinal tract. We have identified B220+ DN T cells as a new type of regulatory T cells that suppress polyclonal T cell activation in vitro and prevent T cell-mediated colitis in the SCID-transfer model of the disease. The mechanism of suppression involves inhibition of IL-2 transcription and inhibition of CD25 upregulation. These findings were obtained by analysis of B220+ DN T cells that accumulate in mice deficient in Fas or FasL genes. However, phenotypically similar B220+DN T cells exist in significant numbers at intraepithelial sites of the appendix, colon, cecum and in the liver, but their function has not been defined. Three Specific Aims are proposed to understand the cellular mechanisms of DN T cell-mediated suppression and the potential role of B220+ DN T cells that reside at the intraepithelial sites of the large intestine and the liver in regulating mucosal tolerance. The three Specific Aims are: 1) Characterize the cellular mechanism by which B220+ DN T cells suppress colitis 2) Define the role of the Fas pathway in controlling regulatory T cell function 3) Characterize the function of intraepithelial (IELs) and hepatic B220+ DN T cells. These studies may provide novel insights into how immune responses of the colon and liver are regulated and mucosol tolerance is maintained. In addition, they lay the groundwork for the analysis of B220+ DN T cell in healthy individuals and IBD patients. [unreadable] [unreadable]